As is well-known, the combustion of fossil fuels (i.e., coal, natural gas, oil) in boilers, furnaces and the like leads to the creation of various pollution components. One of such pollution components is NOx (the various combinations of nitrogen and oxygen, primarily NO and NO2), a pollutant that leads to smog and acid rain, especially in urban environments. Generally, the nitrogen for the formation of NOx comes from air that is introduced into the boiler for combustion, as air consists of approximately 21% oxygen and 79% nitrogen, or from the fuel itself when certain fuels are used. Therefore, when oxygen is burned in a boiler, nitrogen is always present.
At high temperatures the nitrogen will combine with the oxygen to form NOx. Moreover, as the temperatures within the boiler increase, the formation of NOx also increases. At approximately 2200° F. NOx begins to form. As the temperature in the boiler increases beyond this temperature, the formation of NOx increases rapidly. At temperatures over approximately 2780° F. the formation of NOx generally doubles for each increase of about 190° F. This is referred to as thermal NOx. Another type of NOx is known as fuel NOx, in which fuel-bound nitrogen (FBN) is the major source of NOx emissions from combustion of nitrogen-bearing fuels such as heavy oils, coal, and coke. Under the reducing conditions surrounding the burning droplet or particle, the FBN is converted to fixed nitrogen species such as HCN and NH3. These, in turn, are readily oxidized to form NOx if they reach the lean zone of the flame. Under the proper conditions, however, it has been found that these fixed nitrogen species may be converted to molecular nitrogen, thus avoiding the NOx formation path
Systems and methods for reducing various pollution components, such as NOx, produced during combustion processes are known. One example of such a NOx control process is catalytic reduction, in which the relatively cool combustion effluent of several hundred degrees Fahrenheit is passed over a catalyst coated bed in the presence of ammonia. This process is called selective catalytic reduction (SCR). However, while SCR is relatively effective at reducing NOx emissions (it can reduce NOx by more than 90% in some instances), it is a relatively complex process which is relatively expensive both to implement and to operate. Moreover, the more NOx that is present in the combustion effluent, the higher the operating costs for NOx neutralizing processes such as SCR (caused by increased ammonia consumption and decreased catalyst life). Thus, even if a NOx neutralizing process is employed, it would still be desirable to reduce the amount of NOx produced during the combustion step itself.
To date, much of the effort to reduce NOx created during combustion has focused on the design and operation of individual burners themselves. For example, U.S. Pat. No. 4,539,918 discloses a multi-annular swirl burner which includes a plurality of overlapping tubular wall members which form a rich combustion zone and a lean combustion zone with a throat section therebetween. Other references, such as U.S. Pat. No. 4,505,666, relate to burners which employ fuel staging to reduce the amount of NOx produced by the burners. These arrangements are designed to reduce the amount of NOx produced by the burner. However, reducing the amount of NOx produced by individual burners may not provide sufficient NOx reduction when a plurality of such burners are arranged together in a tangential fired boiler. This is true because while the temperature profile in each individual burner may be conducive to producing low NOx, when a plurality of burners are fired together in the boiler, the temperature profile in the boiler may be such that relatively high levels of NOx are produced. Moreover, even though the burners themselves may achieve fuel staging on an individual level, no known systems achieve fuel staging on the furnace level.
U.S. Pat. No. 5,315,939 does disclose an integrated low NOx tangential firing system that is claimed to be particularly suited for use with pulverized solid fuel-fired furnaces, and a method of operating a pulverized solid fuel-fired furnace equipped with an integrated low NOx tangential firing system. However, the system disclosed therein relies upon directional control of the air source. This is disadvantageous in that such systems may not provide adequate NOx reduction.
What is desired, therefore, is a tangential fired boiler which produces relatively low levels of NOx, which produces a relatively low level of NOx during combustion in addition to or instead of neutralizing the NOx produced during combustion, which maintains a temperature profile therewithin that is conducive to producing relatively low levels of NOx, which employs fuel staging on the boiler level rather than merely on the burner level, and which is relatively simple and inexpensive to install and operate.